Off-the-road tire

ABSTRACT

The invention is to improve wear resistance and resistance to crack growth of an off-the-road tire having a gauge of a tread rubber of 60-200 mm, in which a plurality of widthwise grooves extending substantially in a widthwise direction of the tire are arranged in each side region of the tread, and a pair of circumferential fine grooves continuously extending straight-forward or zigzag in a circumferential direction of the tire are arranged in a central zone of the tread, and a plurality of widthwise fine grooves extending substantially in the widthwise direction of the tire and contacting their grooves walls with each other in a ground contact area are arranged between these circumferential fine grooves.

TECHNICAL FIELD

This invention relates to an off-the-road tire capable of simultaneouslyestablishing the wear resistance and resistance to crack growth of atread at a higher level even under severer load condition and tractioncondition.

BACKGROUND ART

Since the off-the-road tires used for construction vehicle and the likeare required to have considerably high load capacity and tractionperformance as compared with tires for truck and bus, it is general tohave a tread pattern that ribs capable of developing a high wearresistance under a high load are formed on a central portion of thetread and lugs capable of developing a high traction performance areformed on both side portions of the tread.

However, when a width of a zone forming straight or zigzag ribs in thecentral portion of the tread sandwiching between phantom lines eachconnecting tops of lug grooves to each other is made wider thannecessary, traction performance during the running on an uphill grade orthe like is lacking, and also there is a drawback that lateral slippageis apt to be easily caused in the steering because circumferentialgroove(s) is not naturally existent in the central portion. For thisend, so-called lug-block mix pattern in which block rows producing anedge effect in a widthwise direction of the tread are formed in thecentral portion of the tread are used at the present time.

On the other hand, the tire is recently and strongly demanded to havefurther improvements of the load capacity and traction performance andan improvement of wear resistance with the further advance of vehicleperformances. In order to satisfy such a demand, there is a tendencythat the tire size is made large and the thickness of the tread is madethicker. In this case, it is particularly feared that heat generation inthe central portion of the tread exerts upon the durability of the tire.

For the purpose of obtaining a tread pattern capable of being durable toexcessive traction and realizing an excellent resistance to lateralslipping and enhancing heat radiation while leaving continuous rib rowsare left in the central portion of the tread as far as possibleconsidering the wear resistance, therefore, it is attempted to arrange apair of circumferential fine grooves for heat radiation in the centralportion of the tread at a required minimum groove width.

In the off-the-road tire provided with the circumferential fine groovesfor heat radiation, however, when the block rows are formed in thecentral portion of the tread as mentioned above, each block easily movesin front and behind or in left and right during the running of the tireunder loading, so that the demand for high load capacity and hightraction performance can not be satisfied and there are caused problemsrelating to the wear resistance such as premature wearing of tread,block chipping and other troubles.

On the other hand, when rows of land portion continuing more thannecessary in the circumferential direction are formed on the centralportion of the tread in such an off-the-road tire, if cut failureproduced on the surface of the land portion row arrives at the beltlayer, a large stress is produced resulting from the fact that the thickland portion rows are continuously arranged in the circumferentialdirection whenever the tread is largely deformed in the circumferentialdirection by subsequently large traction force, whereby there is causeda problem relating to the resistance to crack growth that the cutfailure progresses as a crack between the tread and the belt in thecircumferential direction of the tire along a propagation direction ofsuch stress and further so-called cut separation of completelyseparating the tread from the belt is easily developed.

The invention is to solve the above problems inherent to theconventional technique and to provide an off-the-road tire capable ofproviding a high wear resistance without causing troubles such aspremature wearing of the tread, block chipping and the like even underthe exposure to severer load condition and traction condition, andcapable of providing an excellent resistance to crack growth that evenif cur failure is produced on a ground contact face of the tread, ithardly progresses into a crack between the tread and the belt layer.

DISCLOSURE OF THE INVENTION

The off-the-road tire according to the invention is characterized inthat a gauge of a tread rubber is within a range of 60-200 mm, and aplurality of widthwise grooves extending substantially in a widthwisedirection of the tire are arranged in each side region of the tread, anda pair of circumferential fine grooves continuously extendingstraightforward or zigzag in a circumferential direction of the tire arearranged in a central zone of the tread, and a plurality of widthwisefine grooves extending substantially in the widthwise direction of thetire and contacting their grooves walls with each other in a groundcontact area are arranged between these circumferential fine grooves.

Moreover, the central zone of the tread is a zone corresponding to notmore than 50% of a tread width centering on an equatorial line of thetire.

In this off-the-road tire, a high heat radiation of the tread inaddition to the resistance to lateral slipping can be obtained by thecircumferential fine grooves. On the other hand, the widthwise finegrooves in the central zone of the tread are closed at a state ofcontacting their groove walls with each other during the running of thetire under loading and act as such a land portion that the adjoiningblocks in the circumferential direction of the tire are substantiallycontinued in the circumferential direction, so that the blocks in theblock row located in the central zone of the tread hardly deform infront and behind or in left and right even in the running of the tireunder loading. As a result, the occurrence of troubles such as prematurewearing of the tread, block chipping and the like can be effectivelyprevented to develop an excellent wear resistance even under extremelyhigh load condition and excessive traction condition.

In such an off-the-road tire, the plural widthwise fine grooves arearranged in the central zone of the tread to prevent the land portionrow in the central zone of the tread from the continuing more thannecessary. Therefore, even if cut failure generated in the land portionrow, i.e. the blocks of the block row arrives at the belt layer, thecontinuity of the land portion is prevented by the widthwise finegrooves to reduce stress of strain deformation in the fine grooveportion as compared with that in other portions with respect to a largestrain deformation of the tread in the circumferential direction of thetire, and hence the growth of the cut failure as a crack between thetread and the belt layer in the circumferential direction of the tirecan be prevented by the widthwise fine grooves, so that an excellentresistance to crack growth can also be developed effectively.

According to the off-the-road tire, therefore, the excellent wearresistance can be realized based on the selected width of the widthwisefine groove without bringing about the troubles such as prematurewearing of the tread, block chipping and the like even if the action ofextremely high load and excessive traction is applied to the tire, whilethe excellent resistance to crack growth which hardly progresses the cutfailure produced in the land portion row of the central zone into thecrack between the tread and the belt layer can be realized based on thepresence of the widthwise fine grooves itself.

Preferably, the off-the-road tire is produced by vulcanizing a regionfrom a bead portion of a product tire to a sidewall portion and a treadshoulder portion thereof through a mold part of a full mold form and aregion corresponding to a central portion of the tread located from thetread shoulder portion toward a tread center through a mold part of asplit mold form, respectively.

As the off-the-road tire, there are frequently super-large size tires ofa lug-block mix pattern structure having a thicker tread and relativelydeep circumferential fine grooves in its central portion. In case ofvulcanization building such an off-the-road tire, it is possible toconduct vulcanization through only so-called split mold. In this case,since the tire size is vary large, each segment constituting the splitmold becomes considerably expensive and the manufacture thereof isrequired to take a greater number of steps and also it is difficult torapidly cope with the change of tire size, alteration of pattern and thelike requiring the change of each segment.

On the other hand, when the off-the-road tire is built by vulcanizationthrough only so-called full mold, the exchange, alteration of the pairof the mold parts and the like can be conducted relatively easily, butthere is a fear that when the tire is removed off from the mold, thetread is damaged in the width-narrow circumferential fine grooveportions having a significant groove depth or the like.

In the invention, therefore, the vulcanization is carried out by usingthe mold part of full mold form comprised of two upper and lower moldsegments conducting the mold opening substantially in an axial directionof the tire with respect to the region from the bead portion of theproduct tire to the sidewall portion and tread shoulder portion thereof,and by using the mold part of spilt mold form comprised of pluraldivided segments in the circumferential direction of the tire conductingthe mold opening substantially in the radial direction of the tire withrespect to the region corresponding to the central portion of the treadlocated from the tread shoulder portion toward the tread center, wherebythe number of mold manufacturing steps, the mold cost and the like canbe reduced but also the change of tire size, alteration of pattern andthe like can be conducted quickly and further the fear of damaging thecircumferential groove portions can be sufficiently removed.

In the alteration of the tread pattern of the tire to be vulcanized, itis possible to alter only the mold part of the split mold form while themold part of the full mold form having a high versatility is used as itis, or in other words, the widthwise grooves formed in the treadshoulder zone are common. Thus, a time of exchanging the mold as a timefor waiting the vulcanization can be advantageously shortened and alsothe total cost of the mold can be reduced.

In the invention, a boundary between the mold part of the full mold formand the mold part of the split mold form is made in correspondence witha groove edge of a straight or zigzag circumferential fine groove to beformed in the tread and located toward a side edge of the tread, and thevulcanization is conducted in a mold of these mold parts located alongthe groove edge to produce an off-the-road tire. In the mold opening ofthese mold parts after the vulcanization, a fear of damaging the treadin the vicinity of the groove portion is removed, and also spew rubberoverflowed between these mold parts, which damages the uniformity ofground contact pressure of the land portion to easily generate a nucleusof uneven wear and easily rendered into a nucleus for tread crack as itis, can be positioned on the groove edge, whereby an influence of thespew rubber upon the occurrence of uneven wear and crack can effectivelybe controlled.

When the circumferential fine groove is zigzag, the groove volume perunit circumferential length of the tire can be made larger than that ofthe straight groove to further enhance heat radiation effect through thecircumferential fine groove.

Further, when the off-the-road tire is produced by vulcanizing through amold in which the boundary between the mold parts as mentioned above isstraightforward positioned in correspondence with a groove edge of azigzag circumferential fine groove to be formed in the tread and locatednearest to a side edge of the tread, the damage of the tread in the moldopening after the vulcanization can be prevented but also the spewrubber between these mold parts can be positioned in the land portion ofthe tread side zone producing a relatively low ground contact pressure,whereby the spew rubber resulting in the occurrence of uneven wear andforming the nucleus for crack can be advantageously blocked.

Moreover, when the tire is produced by vulcanizing through a mold inwhich the boundary between the mold parts is positioned incorrespondence with the land portion defined by the circumferential finegroove to be formed in the tread toward the side edge of the tread, thespew rubber between these mold parts is positioned in place having alower ground contact pressure, whereby the fear of resulting in theuneven wear and the like can be sufficiently removed.

On the other hand, the width of the mold part of the split mold formbecomes relatively wider, so that the degree of freedom in thealteration of the tread pattern can be enhanced.

In such an off-the-road tire, when a groove width of the widthwisegroove is 25-80 mm, the traction performance based on the action of thetread side zone and the wear resistance of such a side zone can be wellestablished, while when a groove depth of the widthwise groove is 50-160mm, the wear resistance of the tread side zone and the resistance toheat generation can be well established.

The term “groove width” used herein means an average value of widthsmeasured in an existing direction of a groove center line over 50% of acentral region of the center line, while the term “groove depth” meansan average value of depths measured in an existing direction of a groovecenter line over 50% of a central region of the center line.

When the groove width of the circumferential fine groove is 15-50% ofthe groove width of the widthwise groove, the necessary and sufficientheat radiation effect is ensured but also the rigidity of thecircumferential fine groove forming portion, which corresponds toapproximately a half of a tread half-width and is apt to be relativelyworn, can be enhanced by contacting groove walls with each other in theground contact face, so that the rigidity of the land portion as a wholeof the tread can be increased to improve the wear resistance.

Also, when the groove depth of the circumferential fine groove is60-100% of the groove depth of the widthwise groove, the contacting ofthe groove walls in the ground contact face can easily be realized.

Preferably, the groove width of the widthwise fine groove is 15-50% ofthe groove width of the widthwise groove. In this case, the growth ofcracks between the tread and the belt resulted from the cut failure iseffectively prevented by the widthwise fine grooves, while an excellentwear resistance can be given to the land portion of the central zone ofthe tread by contacting groove walls with each other.

In other words, when the groove width is less than 15%, thecircumferential length of a thin portion beneath the groove as a regionof preventing the crack growth is too short and there is a fear that thewidthwise fine groove can not sufficiently develop an expected function,and also a width of a projection in a mold for the formation of thegroove, i.e. a thickness thereof is too thin and the lowering of thedurability of the projection can not be avoided. While, when it exceeds50%, it is difficult to contact the groove walls in the ground contactface with each other or contact the adjoining land portions in thecircumferential direction with each other and there is left a fear ofchipping the land portions or the like.

Preferably, the groove depth of the widthwise fine groove is 60-100% ofthe groove depth of the widthwise groove. In this case, excellentresistance to crack growth and protecting property to tire casings suchas belt, carcass and the like can be simultaneously realized.

When the groove depth is less than 60%, the rubber gauge of the treadbeneath the widthwise fine groove is too thick, and stress in thecentral zone of the tread during the running of the tire, which resultsin the growth of the cut failure arrived at the belt layer in thecircumferential direction of the tire, can not be sufficiently reduced,so that it is difficult to realize the excellent resistance to crackgrowth. While, when it exceeds 100%, the rubber gauge beneath thewidthwise fine groove is too thin and the tire casings such as belt,carcass and the like can not sufficiently be protected.

In such an off-the-road tire, an average intersecting angle of thewidthwise fine groove at an acute angle side with respect to theequatorial line of the tire is preferably 45-90°.

The term “average intersecting angle” used herein means an angle of aline segment connecting intersects between the circumferential finegroove and the widthwise fine groove to each other with respect to theequatorial line of the tire.

In this case, the excellent wear resistance can be realized. That is,when the intersecting angle is less than 45°, it is difficult to contactthe groove walls of the widthwise fine groove with each other during therunning of the tire under loading and also it is difficult to cooperatethe adjoining land portions in the circumferential direction as a landportion body continuing substantially in the circumferential directionand hence it is difficult to realize the excellent wear resistance.Further, an acute corner part is created in the land portion defined bythe widthwise fine grooves to easily chip such a corner part during therunning of the tire under loading and hence it is difficult to realizethe excellent wear resistance.

Preferably, a JIS A hardness of tread rubber in the central zone of thetread is 55-80, while a loss tangent tan δ (25° C.) thereof is0.05-0.35.

The term “JIS A hardness” used herein means a hardness measured at atesting temperature of 25° C. using a type A testing machine for adurometer hardness test according to JIS K6253-1993, while the term“loss tangent tan δ (25° C.)” means a value that a loss tangent tan δrepresented by a ratio of loss modulus to dynamic modulus is measured ata testing temperature of 25° C. under conditions of initial static loadof 1.6 N, average strain amplitude of 1% and testing frequency of 52 Hzusing a spectrometer made by Toyo Seiki Co., Ltd. according to “casethrough load wave, deflection wave (tension)” of JIS K6394-1995.

In this off-the-road tire, a high rigidity of the land portion isensured and the excellent resistance to crack growth can be provided byrendering the hardness of the tread rubber at the central zone of thetread into 55-80, while the excellent heat resistance can be developedwhile ensuring the necessary rigidity of the land portion by renderingthe loss tangent tan δ (25° C.) into 0.05-0.35.

Preferably, the groove width of the circumferential fine groove is awidth contacting the groove walls with each other in the ground contactface. In this case, the rigidity of portions forming the circumferentialfine grooves and hence the rigidity of the land portions in the whole ofthe tread can be enhanced by contacting the groove walls of thecircumferential fine groove during the running of the tire under loadingas previously mentioned to develop the excellent wear resistance.

Also, it is preferable that the block defined by the circumferentialfine grooves and the widthwise fine grooves as a land portion istriangular, quadrilateral or hexagonal. In this case, the blocks can becontinuously arranged in the circumferential direction to attain auniform block row in the circumferential direction while ensuring theblock rigidity required in the central zone of the tread. In otherwords, a portion having a low rigidity may be produced if the shape ofthe block is complicated.

The off-the-road tire having the above construction is preferable to beproduced by vulcanizing and building in a mold wherein the width of theprojection for the formation of the circumferential fine groove is madesmaller than the width of the projection for the formation of thewidthwise fine groove.

In such an off-the-road tire, it is general to adopt such a beltstructure that the laminating number of belt layers is large in thecentral zone of the tread and small in the side zone thereof for wellestablishing the control of size growth in the central zone of the treadand the prevention on the occurrence of troubles at the side edge of thebelt and the neighborhood thereof. According to this belt structure, aside edge of a narrow-width belt layer contributing to the reinforcementof the central zone of the tread is frequently existent in a positioncorresponding to approximately 25% of a tread width measured from theequatorial plane of the tire or the neighborhood thereof.

For this end, the size growth in the vicinity of the side edge of thenarrow-width belt layer particularly tends to be larger than that of theadjoining portion thereof in the inflation of an air pressure into thetire and during the running of the tire under loading.

Therefore, the groove width of the circumferential fine groove usuallyextending in the vicinity of the side edge of the narrow-width beltlayer and tending to widen the groove width accompanied with the sizegrowth is made approximately equal to the groove width of the widthwisefine groove extending in the central zone of the tread having a smallsize growth in the use of the tire and tending to narrow the groovewidth through the pushing of the surrounding tread rubber, which isattained by selecting the respective widths of the projections for theformation of the respective fine grooves, so that it is preferable thatas to the relative widths of the respective fine grooves at a time ofvulcanizing and building the tire, the circumferential fine groove ismade narrower than the widthwise fine groove considering the influenceof the size growth as mentioned above.

More preferably, the groove bottom of straightforward extending portionsin the zigzag extending circumferential fine groove is rendered into araised bottom at an end portion located toward a side edge of the tread.According to this structure, the increase of the groove width resultedfrom the size growth of the tire can be effectively controlled based onthe increase of the groove bottom rigidity through the raised bottomportion, and also the occurrence of cracks at the groove bottom resultedfrom the large enlargement of the groove width can be prevented.

On the other hand, when the groove bottom of the straightforwardextending portions in the zigzag extending circumferential fine grooveis rendered into a raised bottom at an end portion located toward theside of the tread center, the protrusion of the surface of the landportion at the side edge of the tread outward from the surface of theland portion at the side of the tread center in the radial direction dueto the difference of expansion amount in the radial direction betweenthe land portion located at the side of the tread center and the landportion located at the side of the side edge of the tread, which aredefined by the circumferential fine grooves and resulted from thedifference in the influence of the size growth thereupon, can becontrolled based on the rigidity of the raised bottom portion connectingboth the land portions and hence deformation restraining force.

When a surface height of a corner part in the land portion adjacent tothe side edge of the tread defined by the zigzag extendingcircumferential fine grooves is gradually decreased toward a top sidethereof, even if the surface of the land portion located toward the sideedge of the tread is relatively protruded outward from the surface ofthe land portion located at the side of the tread center in the radialdirection under an influence of the size growth, the formation of thecorner part as a nucleus for uneven wear can be advantageously preventedby decreasing the surface height of the corner part toward the top sidethereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a developed view of a tread pattern showing an embodiment ofthe invention.

FIG. 2 is a diagrammatically section view of a main part in a radialdirection illustrating a boundary position of a mold part.

FIG. 3 is a schematic view illustrating a boundary position of a moldpart in connection with a tread pattern.

FIG. 4 is a developed view of a tread pattern showing another embodimentof the invention.

FIG. 5 is a diagrammatic view illustrating the other embodiment.

FIG. 6 is a diagrammatic view illustrating a further embodiment.

FIG. 7 is a diagrammatic view illustrating a still further embodiment.

FIG. 8 is a developed view of a tread pattern in the conventional tire.

FIG. 9 is a developed view of a tread pattern in another conventionaltire.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 1 is shown a developed view of a tread pattern, in which numeral1 is a tread.

In this embodiment, the tread 1 has a rubber gauge of 60-200 mm, and aplurality of widthwise grooves 2 extending substantially in a widthwisedirection of the tire are arranged in each side zone of the tread 1 anda pair of circumferential fine grooves 3 continuously extending in acircumferential direction of the tire are arranged in a central zone ofthe tread 1, and a plurality of widthwise fine grooves 4 extendingsubstantially in the widthwise direction of the tire and opening to bothcircumferential fine grooves 3 are arranged between both thecircumferential fine grooves. In this case, the groove width of thewidthwise fine groove 4 is a width contacting groove walls with eachother in a ground contact face.

In the illustrating embodiment, the pair of the circumferential finegrooves 3 are extended zigzag in the circumferential direction, but itis possible to extend them in a straight form.

The tire having such a tread pattern can be produced by vulcanizing aregion corresponding to a bead portion and a sidewall portion and a sideportion of a tread of a product tire with a mold part 5 of a so-calledfull mold form and a region corresponding to a central portion of thetread located from the side portion of the tread toward a tread centerwith a mold part 6 of a so-called split mold form as shown, for example,by a radially section in FIG. 2. In this case, it is preferable that aboundary C between the mold part 5 of the full mold form and the moldpart 6 of the split mold form is positioned in correspondence with agroove edge 3 a of the straight or zigzag circumferential fine groove 3formed in the tread 1 and located toward a side edge of the tread andalong the groove edge 3 a.

Moreover, the boundary C between both the mold parts 5 and 6 may bestraightforward positioned in correspondence with a groove edge of thezigzag circumferential fine groove 3 formed in the tread 1 and locatednearest to the side edge of the tread, or positioned in correspondencewith a land portion defined from the circumferential fine groove 3formed in the tread 1 toward the side edge of the tread such as a lug 7.

FIG. 3 is a schematic view illustrating the position of the aboveboundary between the mold parts in connection with a tread pattern,wherein FIG. 3 a shows a case that the boundary C is positioned incorrespondence with the groove edge 3 a of the zigzag circumferentialfine groove 3 located side the side edge of the tread along therewith,and FIG. 3 b shows a case that the boundary C is positioned incorrespondence with the groove edge 3 a of the straight circumferentialfine groove 3 located side the side edge of the tread along therewith.

FIG. 3 c shows a case that the boundary C is straightforward positionedin correspondence with the groove edge 3 a of the zigzag circumferentialfine groove 3 located nearest to the side edge of the tread alongtherewith, and FIG. 3 d shows a case that the boundary C is positionedin the lugs 7 defined from the circumferential fine groove 3 toward theside edge of the tread.

In the thus produced tire, it is preferable that the groove width of thewidthwise groove 2 is 25-80 mm and the groove depth thereof is 50-160mm. Also, it is preferable that the groove width of the circumferentialfine groove 3 is 15-50% of the groove width of the widthwise groove 2and the groove depth thereof is 60-100% of the groove depth of thewidthwise groove 2.

Such numerical ranges for the circumferential fine groove 3 are alsoapplied to the widthwise fine groove 4. That is, it is preferable thatthe groove width of the widthwise fine groove 4 is 15-50% of the groovewidth of the widthwise groove 2 and the groove depth thereof is 60-100%of the groove depth of the widthwise groove 2.

Also, it is preferable that an average intersecting angle θ of thewidthwise fine groove 4 at its acute angle side with respect to anequatorial line E of the tire is 45-90°.

And also, it is preferable that JIS A hardness of a tread rubber in thecentral zone of the tread is 55-80 and a loss tangent tan δ (25° C.) is0.05-0.35.

Furthermore, it is preferable that the groove width of thecircumferential fine groove 3 is rendered into a width of contactinggroove walls with each other in a ground contact face, and that blocks 8in the central zone of the tread defined by the circumferential finegrooves 3 and the widthwise fine grooves 4 are triangular, quadrilateralor hexagonal as illustrated.

According to the above off-the-road tire, the excellent wear resistancecan be developed without causing troubles such as premature wearing,block chipping and the like even at a state of applying an extremelyhigh load and an excessive traction but also the excellent resistance tocrack growth hardly growing from cut failure produced on the surface ofthe block to crack between the tread and the belt layer, and furtherfears such as thermal fatigue and the like due to heat generation of thetread 1 can be sufficiently eliminated by heat radiation of thecircumferential fine grooves 3.

FIG. 4 is a developed view of a tread pattern showing anotherembodiment, in which numeral 12 is a widthwise groove, numeral 13 acircumferential fine groove and numeral 14 a widthwise fine groove.

This is the same construction as in FIG. 1 except that the groove widthof the circumferential fine groove 13 is made wider to a level that thegroove walls are not contacted in the ground contact face and the groovedepth thereof is made shallower than that of FIG. 1 and the groove depthof the widthwise fine groove 14 is made shallower than that of FIG. 1.

Even in this tire, the excellent wear resistance can be developed evenat a state of applying an extremely high load and an excessive tractionlikewise the above case, and also excellent resistance to crack growthand heat radiation function of the tread can be developed.

In the off-the-road tire as shown in FIG. 1, when the groove width ofthe circumferential fine groove 3 is made approximately equal to, forexample, that of the widthwise fine groove 4 at a state of using thetire filled with an air pressure, it is preferable that a width size ofa projection for the formation of the circumferential fine groove in avulcanization mold is made smaller than a width size of a projection forthe formation of the widthwise fine groove considering the amount oftire size growth as previously mentioned. Thus, the groove widths of thecircumferential fine groove and the widthwise fine groove can be easilymade approximately equal based on the fact that the amount of sizegrowth accompanied with the filling of the cir pressure into a producttire is larger in a portion forming the circumferential fine groove thanin a portion forming the widthwise fine groove.

In order to suppress the enlargement of the groove width of thecircumferential fine groove due to the above size growth of theoff-the-road tire, as shown in FIG. 5, it is preferable that a raisedbottom portion 23 c as shown at a section in FIG. 5 b is arranged on anend part of a groove bottom 23 b of a straightforward extending portion23 a in a zigzag extending circumferential fine groove 23 located sidethe side edge of the tread and the rigidity of the groove bottom isenhanced by such a raised bottom portion 23 c.

In case of considering that the size growth of the off-the-road tirebecomes particularly large in the vicinity of the side edge of thenarrow-width belt layer reinforcing the central zone of the tread orfrequently in a portion of the circumferential fine groove adjacent tothe side edge of the tread, as shown in FIG. 6, it is preferable that araised bottom portion 23 d as shown at a section in FIG. 6 b is arrangedon an end part of a groove bottom 23 b of a straightforward extendingportion 23 a in a zigzag extending circumferential fine groove 23located toward the t read center, whereby the protrusion of the surfaceof the lug 27 outward from the surface of the block 28 in the radialdirection is suppressed by the rigidity of the raised bottom portion 23d connecting both to each other. Thus, the protruding amount of the lugsurface to the block surface can be suppressed to a small level.

In stead of the above or in addition thereto, as shown in FIG. 7, when aslant face 27 a gradually decreasing a surface height thereof toward atop side is arranged on a corner part defined by the circumferentialfine groove 23 and adjoining to the side edge of the tread or a cornerpart of the lug 27 in the illustrated embodiment, the protrusion of thesurface of the corner part outward from the block surface is effectivelyprevented by the action of the slant face 27 a and also the rigidity ofthe corner part itself can be enhanced and hence the formation of thecorner part as a nucleus for uneven wear and the premature progress ofthe uneven wear can be prevented advantageously.

EXAMPLE 1

There are prepared off-the-road tires according to the invention andperformance evaluations with respect to wear resistance, resistance tocrack growth, heat radiation and traction performances are carried outas mentioned below.

Example tire 1 has a tread pattern of FIG. 1, and Example tire 2 has atread pattern of FIG. 4, and Conventional tire 1 has a tread pattern ofFIG. 8, and Conventional tire 2 has a tread pattern of FIG. 9,respectively. Also, Example tire 3 has a groove structure of FIG. 5, andExample tire 4 has a groove structure of FIGS. 5 and 6, and Example tire5 has a groove structure of FIGS. 5, 6 and 7.

Further, each of these test tires has dimensions as shown in Table 1, inwhich a region ranging from a bead portion to a sidewall portion has astructure similar to that of an off-the-road tire for a generalconstruction vehicle.

Moreover, each test tire has a tire size of 4000R57, and a rim having awidth of 29 inches and a flange height of 6 inches is used, and an airpressure of the tire is 686 kPa, and a load is 588 kN.

TABLE 1 Example Example Conventional Conventional Example ExampleExample Unit tire 1 tire 2 tire 1 tire 2 tire 3 tire 4 tire 5 Thicknessof tread at equatorial plane of tire mm 110 110 110 110 110 110 110Thickness of tread at ground contact end mm 140 140 140 140 140 140 140JIS A hardness at central zone of tread ° 63 63 63 63 63 63 63 Tan δ(25° C.) at central zone of tread — 0.16 0.16 0.16 0.16 0.16 0.16 0.16Average groove width of widthwise groove mm 70 70 70 70 70 70 70 Averagegroove depth of widthwise groove mm 95 95 95 95 95 95 95 Average groovewidth of circumferential fine groove/ — 0.25 0.45 0.45 — 0.25 0.25 0.25average groove width of widthwise groove Average groove depth ofcircumferential fine groove/ — 0.95 0.85 0.95 — 0.95 0.95 0.95 averagegroove depth of widthwise groove Average groove width of widthwise finegroove/ — 0.25 0.25 0.85 — 0.25 0.25 0.25 average groove width ofwidthwise groove Average groove depth of widthwise fine groove/ — 0.950.85 0.95 — 0.95 0.95 0.95 average groove depth of widthwise grooveAverage intersecting angle of widthwise fine groove ° 70 70 70 — 70 7070 with respect to equatorial line of tire Behavior of circumferentialfine groove in ground — closed not closed not closed — closed closedclosed contact Behavior of widthwise fine groove in ground contact —closed closed not closed — closed closed closed Height of raised bottomportion at side edge of tread mm — — — — 20 20 20 in circumferentialfine groove at tread center mm — — — — — 20 20 Decrease of height incorner part — — — — — absence absence presence

With respect to the wear resistance, each of the tires is mounted onto afront wheel of a construction vehicle and run at an approximately equalspeed of 10 km/h for 1000 hours, and thereafter the tread is dividedinto 8 parts in the widthwise direction of the tire and remaining groovedegrees at these 8 positions are measured to calculate an average valueGw of rubber gauge of the tread worn through the running. A valueobtained by dividing the running time by the average value Gw isevaluated as a value of wear resistance and represented by an index onthe basis that the conventional tire 2 is control.

With respect to the resistance to crack growth, each of the test tiresis mounted onto a driving shaft of a rear wheel of a constructionvehicle and run on an off road at an approximately equal speed of 10km/h for 1000 hours after cut failure reaching to the belt is formed inthe central zone of the tread to measure a length of crack growth. Avalue obtained by dividing the running time by the crack growth lengthis evaluated as a vale of resistance to crack growth and represented byan index on the basis that the conventional tire 2 is control.

With respect to the heat radiation, a hole of about 10 mm in diameterreaching to the belt layer is previously formed at a central position ofa block located at each of 8 positions of the tread in the widthwisedirection of the tire, and then the tire is run on a drum of 5 m indiameter at a speed of 10 km/h for 48 hours, and thereafter atemperature of each hole is measured through a thermocouple. The heatradiation is evaluated by a highest temperature among the measuredvalues.

Moreover, an atmosphere temperature is about 30° C., and the evaluationis carried out according to a standard that the temperature of theconventional tire 2 is 0.

With respect to the traction performance, each of the test tires ismounted onto a driving shaft of a rear wheel of a construction vehicleand run on an off road at an equal speed of 10 km/h from a flat state toa slope having an inclination degree of 8%, during which the tractionperformance is evaluated by 5 stages according to a standard that theconventional tire 2 is 4.

The evaluation results on each performance are shown in Table 2.Moreover, the larger the absolute value of each numeral, the better theresult.

TABLE 2 Example Example Conventional Conventional Example ExampleExample tire 1 tire 2 tire 1 tire 2 tire 3 tire 4 tire 5 Wear 160 135100 100 165 168 170 resistance Resistance to 140 140 130 100 140 140 140crack growth Heat −1 −2 −4 0 −1 −1 −1 radiation Traction 5 5 5 4 5 5 5performance

As seen from Table 2, the wear resistance and the resistance to crackgrowth are considerably excellent in each of the example tires asscompared with each of the conventional tires.

As to the heat radiation, each of the example tires is poor as comparedwith the conventional tire 1, but shows an excellent result as comparedwith the conventional tire 2. This is considered due to the fact thatsince each of the example tires is large in the block volume as comparedwith the conventional tire 1, the quantity of heat generation increasesbut the decrease of the groove volume is not so large and hence the heatradiation is relatively good and the control of the temperature rise isadvantageously realized.

As to the traction performance, each of the example tires shows a resultequal to those of the conventional tires.

EXAMPLE 2

A mold cost and an exchanging time of a split mold part are determinedby changing a kind of molds and a boundary position between mold parts,while an uneven wear of a land portion in a product tire due to thepresence of spew rubbers produced at the boundary position between themold parts is measured at a time of running the tire at a speed of 10km/h over 5000 km to obtain results as shown in Table 3.

In this case, the tire size, use conditions and the like are the same asin Example 1.

TABLE 3 Conventional Comparative Comparative Example Example 1 Example 1Example 2 Example 3 Example 4 Example 5 Example 6 Example 2 Kind of moldfull mold full mold split mold split mold full full full full fullmold + mold + mold + mold + mold + split mold split mold split moldsplit mold split mold Figure corresponding FIG. 9 (lug) FIG. 1 FIG. 1FIG. 1 FIG. 3a FIG. 3b FIG. 3c FIG. 3d FIG. 1 to pattern Boundaryposition none none in center in widthwise widthwise straight at in landin center block shoulder outer edge outer edge outer end portion ofblock block of groove of groove portion of shoulder groove Mold cost *1100 100 250 250 190 170 175 175 175 Exchanging time of — — 100 100  85 85  85  85  85 split mold part *2 Take-out of possible impossiblepossible possible possible possible possible possible impossiblevulcanized tire Uneven wear *3  94 impossible  94  94 100 100  97  94impossible (influenced by spew) to produce to produce tire tire *1: Thesmaller the value, the better the cost. *2: In case of changing patternby exchanging only split mold part, the smaller the value, the betterthe time. *3: Uneven wear: ratio of worn amounts in land portions atboundary position (excluding cut portion and the like) (worn amount ofminimum worn portion/worn amount of maximum worn portion) of 100 isgood, while small value thereof is bad.

As seen from Table 3, the exchanging of the split mold part is excellentin all examples though there is a disadvantage in the mold cost, andalso the smooth taking-out of the vulcanized tire can be attained.Particularly, the uneven wear due to the presence of spew rubbers can beeffectively prevented in Examples 1, 2 and 6.

INDUSTRIAL APPLICABILITY

According to the invention, the wear resistance and the resistance tocrack growth in the off-the-road tire can be improved while ensuring theexcellent heat radiation.

1. An off-the-road tire having a gauge of a tread rubber within a rangeof 60-200 mm, in which a plurality of widthwise grooves extendingsubstantially in a widthwise direction of the tire are arranged in eachside region of the tread, and a pair of circumferential fine groovescontinuously extending zigzag in a circumferential direction of the tireare arranged in a central zone of the tread, and a groove bottom of onlyan end portion of a straightforward extending portion in the zigzagcircumferential fine groove is rendered into a raised bottom at an endportion located toward a side edge of the tread and/or an end portionlocated toward the side of the tread center, and a plurality ofwidthwise fine grooves extending substantially in the widthwisedirection of the tire and contacting their groove walls with each otherin a ground contact area are arranged between these circumferential finegrooves, and wherein the groove width of the circumferential fine grooveis rendered into a width contacting the groove walls with each other ina ground contact face.
 2. An off-the-road tire according to claim 1,wherein the widthwise groove has a groove width of 25-80 mm.
 3. An offthe-road tire according to claim 1, wherein the widthwise groove has agroove depth of 50-160 mm.
 4. An off-the-road tire according to claim 1,wherein a groove dept of the circumferential fine groove is 60-100% ofthe groove depth of the widthwise groove.
 5. An off-the-road tireaccording to claim 1, wherein the groove width of the widthwise finegroove is 15-50% of the groove width of the widthwise groove.
 6. Anoff-the-road tire according to claim 1, wherein the groove depth of thewidthwise fine groove is 60-100% of the groove depth of the widthwisegroove.
 7. An off-the-road tire according to claim 1, wherein an averageintersecting angle of the widthwise fine groove at an acute angle sidewith respect to the equatorial line of the tire is 45-90°.
 8. Anoff-the-road tire according to claim 1, wherein a tread rubber in thecentral zone of the tread has a JIS A hardness of 55-80 and a losstangent tan δ (25° C.) of 0.05-0.35.
 9. An off-the-road tire accordingto claim 1, wherein each of blocks defined by the circumferential finegrooves and the widthwise fine grooves as a land portion is triangular,quadrilateral or hexagonal.
 10. An off-the-road tire according to claim1, wherein a surface height of a corner part in the land portionadjacent to the side edge of the tread defined by the zigzag extendingcircumferential fine grooves is gradually decreased toward the zigzagcircumferential fine grooves.
 11. An off-the-road tire according toclaim 1, wherein a groove width of the circumferential fine groove is15-50% of the groove width of the widthwise groove.
 12. A method ofproducing an off-the-road tire having a gauge of a tread rubber within arange of 60-200 mm, in which a plurality of widthwise grooves extendingsubstantially in a widthwise direction of the tire are arranged in eachside region of the tread, and a pair of circumferential fine groovescontinuously extending zigzag in a circumferential direction of the tireare arranged in a central zone of the tread, and a groove bottom of onlyan end portion of a straightforward extending portion in the zigzagcircumferential fine groove is rendered into a raised bottom at an endportion located toward a side edge of the tread and/or an end portionlocated toward the side of the tread center, and a plurality ofwidthwise fine grooves extending substantially in the widthwisedirection of the tire and contacting their grooves walls with each otherin a ground contact area are arranged between these circumferential finegrooves, comprising: vulcanizing a product tire to produce saidoff-the-road tire through a mold in which a width of a projection forthe formation of the circumferential fine groove is made smaller than awidth of a projection for the formation of the widthwise fine groove.13. A method of producing an off-the-road tire according to claim 12,wherein a region corresponding to a bead portion and a sidewall portionand a tread shoulder portion of a product tire is vulcanized to producesaid off the-road tire through a mold part of a full mold form and aregion corresponding to a central portion of a tread located from thetread shoulder portion toward a tread center is vulcanized through amold part of a split mold form, respectively.
 14. A method according to13, wherein a boundary between the mold part of the full mold form andthe mold part of the split mold form is made in correspondence with agroove edge of a zigzag circumferential fine groove to be formed in thetread and located toward a side edge of the tread, and the vulcanizationis conducted in a mold of these mold parts located along the grooveedge.
 15. A method according to 13, wherein the vulcanization is carriedout though a mold in which a boundary between the mold part of the fullmold form and the mold part of the split mold form is straightforwardpositioned in correspondence with a groove edge of a zigzagcircumferential fine groove to be formed in the tread and locatednearest to a side edge of the tread.
 16. A method according to 13,wherein the vulcanization is carried out through a mold in which aboundary between the mold part of the full mold form and the mold partof the split mold form is positioned in correspondence with a landportion defined by the circumferential fine groove to be formed in thetread toward the side edge of the tread.